1. Field of the Invention
The present invention relates to a mechanism for detecting the size of a paper sheet stored in a paper feed tray of an image forming apparatus.
2. Description of the Related Art
An image forming apparatus generally includes box shaped paper feeding trays for storing or stacking paper sheets of various sizes. A paper feeder picks up an appropriate paper sheet and feed the paper sheet to relevant parts of the image forming apparatus.
In conventional image forming apparatuses, a user indicated the size of paper sheets loaded in the feeding tray with a manual operation. However, if the user makes an error, the error cannot be confirmed by the image forming apparatus, it results in a paper jam. Japanese Patent Laid-Open Publication No H11-165881 and Japanese Patent Laid-Open Publication No 2002-187626 disclose paper feeders that can automatically detect the paper size. However, these paper feeders can detect the paper size only in the paper feed direction, but not in the paper width direction. Although some paper feeders detect the paper size both in the paper feed direction as well as in the paper width direction, commonly used paper feeders can only approximately detect the paper size. This results in limitations on the detected paper sizes and a need to provide a plurality of paper size detecting sensors, thus increasing the cost.
The image forming apparatus is generally provided with a paper separating member that separates the top sheet from the other sheets in the paper feeding tray before feeding the top sheet. The paper separating member includes a feed roller that is stopped from rotation immediately when there is a failure in identification of the paper size or an error in setting of the paper size. This results in an increase in slipping of paper sheets, thereby lowering the accuracy of paper sheet transfer, and affecting productivity.
To overcome the aforementioned drawbacks, an automatic paper size detecting tray has been developed. Such a paper size detecting tray includes a lever that detects the paper size. The lever engages with a regulating member that regulates the paper in a paper width direction. For the sake of convenience for the user, the regulating member and a lock are preferably positioned at the front side of the paper size detecting tray. However, a paper size detecting switch to be pressed by the lever is positioned at the back side of the paper size detecting tray. Therefore, the lever is inevitably engaged with the regulating member at the back side. When the lever is engaged with the regulating member at the front side, shaking of a rack and a pinion and precision accumulation from other units are caused. This causes a shift in the position of the regulating member engaged with the lever, which leads to inaccurate paper size detection.